1. Field of the Invention
This invention relates generally to the combustion system of a gas turbine engine and, more particularly, to a double annular combustor having concentrically disposed inner and outer annular combustors with inner and outer domes, and a one-piece cowl extending upstream of and connected to the inner and outer domes having an inner annular portion, an outer annular portion, and a radial middle portion connecting the inner and outer annular portions.
2. Description of Related Art
Efforts to reduce emissions in gas turbine engines have brought about the use of staged combustion techniques wherein one burner or set of burners is used for low speed, low temperature conditions such as idle, and another, or additional, burner or burners are used for high temperature operating conditions. One particular configuration of such a concept is that of the double annular combustor wherein the two stages are located concentrically in a single combustor liner. Conventionally, the pilot stage section is located concentrically outside and operates under low temperature and low fuel/air ratio conditions during engine idle operation. The main stage section, which is located concentrically inside, is later fueled and cross-ignited from the pilot stage to operate at the high temperature and relatively high fuel/air ratio conditions. The swirl cups of the respective pilot and main stage sections generally lie in the same radial and circumferential planes, as exemplified by U.S. Pat. No. 4,292,801 to Wilkes, et al. and U.S. Pat. Nos. 4,374,466 and 4,249,373 to Sotheran.
However, as discussed in a development report to the National Aeronautics and Space Administration (NASA) on combustion system component technology for the Energy Efficient Engine (E.sup.3) and U.S. Pat. No. 4,194,358 to Stenger, the pilot stage and the main stage may be radially offset (i.e., lie in distinct radial planes). In both the '358 patent and E.sup.3 configurations, the effective length of the main stage section is relatively short and the effective length of the pilot stage section is relatively long. This configuration allows for complete or near-complete combustion to reduce the amount of hydrocarbon and carbon monoxide emissions since there is a relatively long residence time in the pilot stage section and a relatively minimal residence time in the main stage section.
Whether the inner and outer combustors are radially aligned or not, and whether the outer annular combustor acts as the pilot stage or main stage, the prior art discloses the use of cowls immediately upstream of the dome region to control air flow coming into the combustor. This air flow is divided into three streams: an inner stream which flows into an inner liner passage defined by an inner liner of the combustor and a shell surrounding the liner; an outer stream which flows into an outer liner passage defined by an outer liner of the combustor and a shell surrounding the liner; and a center stream which flows into the combustor dome region defined by inner and outer cowling. Generally speaking, pressure losses occur in the regions where air flow is dumped into the liner passages. However, free stream diffusion occurs in the center stream upstream of the combustor cowlings, which produces a pitot tube effect for the air flow and results in low pressure losses in the region inside the combustor cowlings.
In applications where multiple combustor domes are used in conjunction with a prediffuser of conventional design, cowls which are designed to attain free stream diffusion tend to interfere with the insertion and removal of fuel tubes and nozzles and therefore require notches in the cowling known as "scallops." Such an arrangement is unacceptable due to an increase in pressure loss which occurs ahead of the domes and in the liner passages as air flow passes through the cowling as intended, but then passes back out through the scallops into the liner passages. Moreover, cowls which have been designed so as not to interfere with fuel tubes and nozzles (e.g., cowls having shorter length which do not extend as far upstream) also result in unacceptable pressure losses due to excessive flow spillage from inside the cowl caused by improper sizing of the cowl open area.
Accordingly, the present invention proposes a cowl design for combustors having multiple domes which eliminates the aforementioned problems of cowls heretofore known.